Technically speaking, any RV can become fuel efficient depending on how well you take care of it. Making sure that you regularly check on the engine and the tires is one way for it to be fuel efficient. Weight is also another factor. Of course, the heavier your RV with all the stuff you have here and the things that you pack, the less efficient it is.

I agree! Taking care of your RV, whichever model it is, can easily make it fuel efficient. It also pays to take a look where and when you're filling up. Based on experience, I discovered that you get more value for your money if you fill up during the weekdays rather than the weekends since the price of fuel tends to be more expensive during the weekends. Try also filling up at night. Pumps tend to pump in more gas into your RV when the temperatures are lower.

That being said, I would recommend that you ask your friend to look at other factors when choosing which RV she is planning to get, like space requirement, ease of handling while driving, etc.